Method and Device for Creating a Cephalometric Image

ABSTRACT

An extra-oral dental imaging system comprises an X-ray source ( 102 ) and an imaging device ( 101 ) suitable for producing multiple frames during at least part of an exposure of an object ( 200 ), the imaging device ( 101 ) being displaced along a scanning direction (X). A method for creating a cephalometric image of a human skull comprises a step of setting said imaging device ( 101 ) with an active area having in an imaging plane a width extending along said scanning direction (X), said width varying along a height direction perpendicular to said scanning direction (X); a step of synchronously displacing the X-ray source ( 102 ) and the imaging device ( 101 ) along said exposure profile; and a step of registering multiple frames produced by the imaging device ( 101 ) during the exposure of said object ( 200 ) to be imaged. Using for creating a cephalometric image by digital tomosynthesis.

FIELD OF THE INVENTION

The present invention concerns a method for creating a cephalometricimage as well as a related system.

The present invention relates to the field of dental extra-oral imagingsystems.

More specifically, the present invention relates to cephalometricimaging, namely a linear projection of the human skull or part of thehuman skull.

BACKGROUND

The 2D cephalometric radiograph is an imaging technique which produces alinear projection of a human head on a flat 2D sensor (or, in moregeneral terms, an imaging layer of an imaging device).

Cephalometric analysis is a technique commonly employed byorthodontists, dentists, et al. to analyze the dimensional relationshipsin the craniofacial complex, to predict future changes, to assess theeffect of ongoing treatment plans, to evaluate the patient'sdentomaxillofacial proportions, and to aid in the diagnosis ofabnormalities and asymmetries.

Consequently, there is a need for a system which produceshigh-resolution cephalometric images.

A planar imaging system has two types of resolution: an in-plane spatialresolution, in the direction parallel to the imaging layer of theimaging device; and a depth resolution, perpendicular to the imaginglayer of the imaging device.

The image depth resolution depends primarily on the width of the sensoralong the direction of movement and the actual movement trajectory.

In the known extra-oral systems for performing panoramic imaging, thesensor is typically narrow in aspect, long and with a small width. Inorder to obtain a linear projection of the entire human skull(cephalometric image), the imaging system includes a cephalometric or“ceph” arm: the panoramic sensor is attached on the “ceph” arm in thecases where a cephalometric image is needed.

As a consequence, a large distance is provided between the X-ray sourceand the sensor, in order to minimize distortions and magnificationdisproportions in the projected image. Such a cephalometric arm iscumbersome. Moreover, this technique creates a cephalometric image whereboth the left and right sides of the patient's cranium are superimposedat different degrees of magnification, which can result in imagedistortion and may be of limited diagnostic and therapeutic utility.

The document U.S. Pat. No. 8,306,181 discloses an extra-oral dentalimaging system with a sensor capable of producing a cephalometric imagewithout the use of an additional “ceph” arm.

An elongated rectangular sensor is used with an aspect ratio m:nsuperior to 1.5, wherein m is the long dimension of the active area ofthe sensor and n is the short dimension of the active area of thesensor.

The sensor and the X-ray source are displaced together along atrajectory divided into three segments: a first exposure, anon-radiating movement and a second exposure. During the two exposuresegments, the left and right side of the skull are imaged.

The two segments of the profile during which the skull is exposed toradiation are substantially linear. The length of the linear exposure isgenerally more than 5 cm, but in any case long enough to produce datathat can be used to produce a volumetric reconstruction of an image tobe displayed.

The distance between the focal point of the X-ray source and the imagingdevice is small compared to the standard case with the “ceph” arm.

After the exposures, a volumetric reconstruction algorithm is used tocalculate vertical slices along the imaging direction. The content ofeach individual slice is reconstructed using tomosynthesis techniques.The vertical slices are then transformed to eliminate differentmagnification factors of different vertical slices, and then addedtogether to produce a 2D cephalometric image or 2D cephalogram.

A volume reconstruction with an acceptable depth resolution necessitatesthe use of a sensor with an active area that is sufficient in thescanning direction. By “active area,” it is meant the area of the sensorwhich is irradiated during the scan and which actually participates inthe image reconstruction process.

It has been observed that the use of an active area in the shape of anarrow strip is inappropriate for realizing a three-dimensionalreconstruction of acceptable quality. On the other hand, the larger thesize of the x-ray sensor in the scanning direction, the larger the x-raydose must be administered to the patient. In other words, having a highdepth resolution is disadvantageous in that it requires a high x-raydosage to the patient.

It is noted that what constitutes a sufficient depth resolution dependson the part of the body being imaged. However, certain parts of thebody, either because they are not of primary importance from adiagnostic or therapeutic standpoint or because they are simply notcomplex structures, do not need to be reconstructed with a high level ofdetail. For example, while the mandibles and dentition must be imagedwith a great deal of detail, the rest of the skull can be imaged at amuch lower level of detail and still produce a useful image. Such partsgenerally comprise a lower number of anatomical “landmarks,” and thus alower depth resolution is satisfactory.

There is thus a need for a system and method for imaging that is able toadapt the depth resolution, and by extension the local radiation dose,to the structure of the irradiated anatomy.

The present invention aims to address, in whole or in part, at least theforegoing and other deficiencies in the related art.

SUMMARY OF THE INVENTION

According to one aspect of the disclosure, there is provided a methodfor creating a cephalometric image of at least part of a human skull inan extra-oral dental imaging system, said system comprising an X-raysource for irradiating an object to be imaged; an imaging devicesuitable for producing multiple frames during at least part of anexposure of said object; a manipulator for displacing the imaging devicealong an exposure profile between multiple frames during said at leastpart of the exposure of said object, the manipulator permitting themovement of the imaging device along a scanning direction.

According to the invention, said method comprises steps of setting saidimaging device with an active area having in an imaging plane a width,extending along said scanning direction, said width varying along aheight direction perpendicular to said scanning direction; synchronouslydisplacing the X-ray source and the imaging device along said exposureprofile; and registering said multiple frames produced by the imagingdevice during the exposure of said object to be imaged.

Due to the use of an imaging device with an active area varying in theheight of the imaging device, the depth resolution may be adapted to theregions of interest of a human skull for creating a cephalometric image.

Such an adapted active area of the imaging device provides a goodcompromise between the depth resolution obtained in a directionperpendicular to the plane of the image (needed for creating thecephalometric image), and the X-ray dose on the patient skull.

Thus, thanks to the 3D information, an accurate landmarking may begenerated and precise reference points may be used for the cephalometricanalysis. Preferably, said active area is symmetric in said imagingplane, with a central axis extending along said height direction of saidactive area.

This is advantageous in that the method can be executed to scan fromboth the left and right sides of the head, without any deformation orvisual artifacts provoked by the asymmetry of the active area.

In a possible embodiment, said active area has at least two portionshaving widths different from each other, said two portions beingsuperposed in said imaging plane along said height direction.

In possible embodiments, the height of the active area is between 120 mmand 280 mm, with the width of the first portion being between 2 mm and50 mm and the width of the second portion being between 50 mm and 140mm.

In an advantageous embodiment, said active area has at least threeportions with respectively three different widths.

This is advantageous in that, through the use of at least three activeportions, the depth resolution of the scan is more closely tailored tothe anatomy being scanned.

Most preferably, said active area has a central portion, a lowerportion, and an upper portion each extending along said height directionin said imaging plane, the width of the central portion is larger thanthe width of the lower portion and the width of the lower portion islarger than the width of the upper portion.

The three portions can be made to correspond to three regions of theanatomy for which cephalographs are of particular interest: the cranium,the jaw and dentition, and the chin and vertebrae.

Since each of these regions has a different level of structuralcomplexity, the widths of each of the portions can be made to correspondto the desired level of depth resolution for the region to which itcorresponds.

Such a configuration of the active area gives a good differentiation indepth resolution, along with acceptable levels of X-ray dosing, for manymedical applications.

In a possible embodiment, the method further comprises a step ofcomputing the multiple frames produced during at least one part of theexposure by a shift-and-add processing, thereby reconstructing at leastone slice; or by a volumetric approach, thereby reconstructing athree-dimensional volume and subsequently extracting at least one slicefrom said volume; said at least one slice from said volume containingin-focus imaging data belonging respectively to at least one depth ofsaid object to be imaged.

In this way, a three-dimensional model of the patient's anatomy isconstructed, providing high-resolution, high-precision information thatis not limited to a narrow “focal trough” but is rather of aconsistently high quality through the entire depth of the scan.

Moreover, from this model, high-quality simulations of conventionalimaging scans can easily be extracted and/or extrapolated, maximizingthe diagnostic and therapeutic utility of each scan.

In possible embodiments, the volumetric approach is selected from aStatistical Algebraic Reconstruction Technique (SART), a StatisticalIterative Reconstruction Technique (SIRT), or a Filtered Back ProjectionTechnique.

Such approaches are advantageous in that they yield a high-qualityreconstruction of the subject while limiting the X-ray dosage incurred.In particular, certain a priori information is employed so as to refinethe reconstruction, such as the positions of certain anatomical featuresin the patient's cranium. This in turn refines the reconstruction andimproves the quality of the images produced from it.

Preferably, the method further comprises a step of using eachreconstructed slice for the extraction of cephalometric features.

Most preferably, in a step of automatic cephalometric tracing, saidextracted cephalometric features of each slice are put together.

In this way, a complete cephalometric image is constructed, therebyenabling e.g. a dentist to perform diagnostic and therapeutic proceduresbased thereupon.

In a possible variant embodiment, several slices are reconstructed andcombined to give a separate linear projection for the left and rightsides of said object to be imaged.

In this way, a pair of cephalometric images is constructed with a singlescan, reducing X-ray dosage to the patient while providing bilateralcephalometric information.

In another possible variant embodiment, several slices are reconstructedand retro-projected to a distance superior to 1.50 meters, andpreferably superior to 4 meters, on a cone beam or parallel geometry soas to create a synthesized 2D cephalogram of the skull.

This is advantageous in that a traditional 2D cephalogram is produced,without requiring an extra “ceph arm” or the space to accommodate it, orany additional X-ray exposure to the patient.

According to another embodiment, the method for creating a cephalometricimage of a part of a human skull comprises the following steps:

-   -   synchronously displacing the X-ray source and the imaging device        along a first part of said exposure profile, said X-ray source        being in an upper position along said height direction;    -   registering said multiple frames produced by the imaging device        during said first part of the exposure profile;    -   synchronously displacing the X-ray source and the imaging device        along a second part of said exposure profile, said X-ray source        being in a lower position along said height direction;    -   registering said multiple frames produced by the imaging device        during said second part of the exposure profile; and    -   combining said multiple frames registered during said first and        second parts of the exposure profile.

Thus, the patient skull is scanned with an X-ray source at severaldiscrete positions along the height direction.

According to another aspect, there is provided an extra-oral dentalimaging system for creating a cephalometric image of at least part of ahuman skull, such system comprising an X-ray source for irradiating anobject to be imaged, an imaging device suitable for producing multipleframes during at least part of an exposure of said object; a manipulatorfor displacing the imaging device along an exposure profile betweenmultiple frames during said at least part of the exposure of saidobject, the manipulator permitting the movement of the imaging devicealong a scanning direction

According to the invention, said imaging device has an active areahaving in an imaging plane a width extending along said scanningdirection, said width varying along a height direction perpendicular tosaid scanning direction; and said manipulator synchronously displacesthe X-ray source and the imaging device along said exposure profile; andsaid imaging device comprises memory for registering said multipleframes produced by the imaging device during the exposure of said objectto be imaged.

Such an apparatus is advantageous in that it realizes the advantageousaspects of the method and variants described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the embodiments of the invention, as illustrated in theaccompanying drawings.

The elements of the drawings are not necessarily to scale relative toeach other. Some exaggeration may be necessary in order to emphasizebasic structural relationship or principles of the invention. Someconventional components that would be needed for implementation of thedescribed embodiments, such as support components used for providingpower, for packaging, and for mounting and protecting x-ray systemcomponents, for example, are not shown in the drawings in order tosimplify the description.

FIG. 1A is a diagram showing a perspective view of an extra-oral dentalimaging system according to a first embodiment of the invention;

FIG. 1B is a detail of an alternate configuration of a patient supportfor the imaging system of FIG. 1A;

FIG. 1C is a representation of the voxel size and shape realized by themethod of the invention and a scan according to a CBCT method of theart;

FIG. 2 is a diagram showing an exemplary exposure profile performed in amethod for creating a cephalometric image according to a firstembodiment of the invention;

FIGS. 3A and 3B are diagrams showing exemplary active areas of animaging device provided in the method for performing a cephalometricimage according to the first embodiment of the invention;

FIG. 4 is an illustration of voxels produced by imaging device accordingto the first embodiment of the invention, superimposed onto a humanskull;

FIG. 5 is an illustration of voxels produced by an imaging deviceaccording to a first embodiment of the invention, compared with voxelsproduced by the same technique with a narrow rectangular sensor;

FIG. 6 is an illustration of a collimator for an imaging deviceaccording to a first embodiment of the invention;

FIG. 7 is an exemplary configuration for a gantry of the imaging systemdepicted in FIG. 1A;

FIG. 8 is a flow chart that shows an exemplary method for performing acephalometric image according to one embodiment of the invention;

FIG. 9 is an example of the in focus features extracted from a slice onthe zero sagittal plane of a human skull; and

FIG. 10 is an example of the superimposition of the in focus featuresextracted from all slices belonging to a left part of a human skull; and

FIG. 11 is an alternative embodiment of an exposure profile performed ina method for creating a cephalometric image.

FIG. 1A illustrates the general configuration of an extra-oral dentalimaging system 100 according to a first embodiment of the invention. Theimaging system 100 comprises a sensor 101 and an X-ray source 102. Thesensor 101 and the X-ray source 102 are mounted in this embodiment on agantry 103 which is itself fastened to a horizontal mount 104.

The horizontal mount 104 is fixed to a vertical column 105 which maycomprise classical telescopic means not disclosed here below, permittingto set the height of the imaging system.

The imaging system also comprises a patient holder 106 which maintainsthe patient head in a defined and fixed position under the gantry 103,between the X-ray source 102 and the sensor 101 during the imagingprocess.

The patient holder 106 may be similar to a patient holder used, in priorart, on a cephalometric imaging arm to maintain the patient head duringthe exposure.

As an alternative embodiment, the patient holder 106 may be composed oftwo ear rod holders 108 supporting two ear rods 109 to be inserted inthe patient's ear canals 201, as illustrated in FIG. 1B. The two ear rodholders are symmetrically slideable on each side to adjust to the widthof the patient's head 200.

A nosepiece 110 is adjustable in the horizontal and vertical directionsto be positioned exactly at the bridge (nasion) of the patient's nose202. The ear rods 109 inserted into the ear canals 201 of the patientblock any possible movement of the patient's head, except the rotationof the head about an axis passing through the two ear rods 109.

The nosepiece 110 thus serves to prevent such movement, constraining thepatient's movement in this last degree of freedom. A mount 107 supportsthe ear rod supports 108 and the nosepiece 110, and can be fitted on themount 104 of the imaging device through the gantry 103.

In an alternative embodiment, the patient holder 106 can be fixed withan arm (not represented on the vertical column 105); this will allow, incertain situation, a greater deal of freedom in the configuration andoperation of the imaging system.

The X-ray source 102 is adapted to irradiate the object to be imaged,and in this embodiment, at least a part of a human skull for creating acephalometric image.

The sensor 101 forms an imaging device suitable for producing multipleframes during the exposure of the object to be imaged.

In one embodiment, it is envisioned that the X-ray sensor 101 is acharge-coupled device (CCD), a CMOS sensor, or a TFT sensor, as such adevice could be easily integrated into a computerized imaging systemwith minimal adaptation.

The gantry 103 forms a manipulator for displacing the sensor 101 and theX-ray source 102 along an exposure profile.

Thus, the manipulator or gantry 103 permits the movement of the X-raysource 102 and the sensor 101 by means of a selective translation and aselective rotation.

FIG. 1C illustrates the result of the invention as compared to that of aCone-Beam Computerized Tomography (CBCT) technique as known in the art.The grid 150 is a representation of the result of the method of thepresent invention, while the grid 160 is representative of the result ofthe CBCT method. In particular, it can be seen that the area in the x-yplane of each of the “blocks” 151 is very small compared to the blocks161, meaning that the in-plane resolution of the method according to thepresent invention is very high compared to that of the CBCT method.

On the other hand, the depth resolution is not as fine (as can be seenin the increased height of the grid 150 relative to the grid 160).However, for cephalometric purposes the depth resolution of the methodof the present invention is nonetheless acceptable, and the x-ray dosageremains very low compared to CBCT.

Turning now to FIG. 2, the exposure profile of the imaging system 100 isillustrated in greater detail.

The exposure profile is performed for instance when creating acephalometric image of a part of human skull.

During the exposure of the patient 200, the X-ray sensor 101 is suitablefor producing multiple frames.

As known in an extra-oral dental imaging system, it comprises a memoryfor registering the frames produced by the imaging device.

In this embodiment, the imaging system comprises a memory forregistering the multiple frames produced by the X-ray sensor 101 duringthe exposure of the patient skull.

The imaging system 100 is first positioned about a patient 200, suchthat the sensor 101 and the X-ray source 102 are disposed in an initialposition A. The initial position A of the X-ray sensor 101 and the X-raysource 102 are such that a line drawn between them lies just forward ofthe face of the patient 200.

The X-ray sensor 101 and the X-ray source 102 are swept past the patient200 as displayed in FIG. 2; more specifically, the X-ray sensor 101 andthe X-ray source 102 are shown in the initial position A mentionedabove, a final position C, and an intermediate position B between theinitial position A and the final position C.

The effect of this displacement is twofold: the X-ray sensor 101 and theX-ray source 102 are rotated through an angle θ, while simultaneouslytranslating along an X-axis from the initial position A to the endposition C.

Thus, the X-ray source 102 and the X-ray sensor 101 are synchronouslydisplaced along the exposure profile as depicted in FIG. 2. The exposureprofile comprises at least one substantially linear section between theinitial position A and the end position C.

As illustrated in FIG. 2, the X-ray source 102 and the X-ray sensor 101are translated while rotating in the same direction. The angular range θof the rotational movement of the sensor 101 and the X-ray source 102 isat least equal to 15°, and preferably equal to 30°.

In this way, thanks to the limited rotation of the sensor 101 and theX-ray source 102, the profile of the patient is irradiated and imagedwith an almost perpendicular angular incidence; the curvature of thetrajectory of the x-ray source 102 and the x-ray sensor 101 areexaggerated here for illustrative purposes. The trajectory of the gantrycan be considered in gross as being a substantially linear path, in thatthe rotation of the gantry is minimal relative to its translation.

As can be seen in FIG. 2, the patient is positioned close to the sensor101, so that the distance between the sensor 101 and the patient 200 islower than the distance between the patient 200 and the x-ray source102. This limits the distortion of the image due to the conical shape ofthe x-ray beam emitted by the x-ray source 102.

The curvature of the trajectory of the x-ray source 102 is directedtowards the patient 200, and the curvature of the trajectory of thesensor 101 is directed away from the patient. There thus exists aninstantaneous center of rotation that is located on the other, side ofthe sensor 101 from the x-ray source 102. As the patient 200 is locatedclose to the sensor 101, the distance between the patient and theinstantaneous center of rotation is minimized when the instantaneouscenter of rotation is on the side of the sensor 101, as illustrated inFIG. 2, resulting in a higher depth resolution.

For any volume element (voxel) position, it is ideal that the averagedirections of all rays passing through said position to match as closelyas possible the ray direction of a true 2D cephalometric image, i.e.that which would be produced by an apparatus with the sensor placed atthe end of the long “ceph arm” mentioned above. This in turn means thatthe tomosynthesis blur function will have minimal magnification anddistortion artifacts.

This top view of the system does not show the path of the x-rays in theZ-direction. The average ray direction of CBCT x-rays will have aZ-component whose magnitude increases with distance from the primaryplane. This means that the blur (associated with the limitedtomosynthetic depth resolution) will “smear” anatomic features in adirection other than the ray direction of true two-dimensionalcephalometric rays. The direction of this smear will align less closelywith the true (and desired) 2D cephalometric ray direction as the rayspass through anatomy that is further form the plane and closer to thehead and neck of the patient. Without highly accurate (i.e. fine) depthresolution, this will lead to blurring in a simulated cephalometricreprojection of the data outside of a very thin focal trough.

In the CBCT process, this depth resolution is created from theinformation captured by multiple x-ray beam paths through each voxelfrom the CBCT projections. If the sensor 101 were very large such thatthe entire head, or nearly the entire head, were captured in eachprojection, there would be a lot of flexibility allowed in the sequenceof projections and the capture geometry used. The medium-sized sensor101 of the apparatus 100 limits the geometry of the capture assubsections of the anatomy must be captured with each projection andthen combined by image-processing methods to “stitch” together the imageof the entire head.

In the present invention, however, rotation through the scan tilts thesensor 101 so that it is not contained within a single plane. A centerof rotation 203 (numbered here as 203A, 203B, and 203C to correspond tothe three positions A, B, and C of the gantry 103) is moved with an Xcomponent (along the sagittal plane of the patient 200). The center ofrotation 203, in addition to moving in the X-direction (i.e. thesagittal direction), also moves a much smaller amount in the Z-direction(i.e. the coronal direction) in the case of a cephalometric scan asdepicted in FIG. 2. Moreover, the center of rotation 203 is positionedabove the patient's head 200 and on the same side of the X-ray source102 (in FIG. 2, the left-hand side).

The general “convex” scan path illustrated in FIG. 2 balances all ofthese requirements. The exact scan path will depend, of course, on thesystem components and imaging requirements, such as any constraints onthe movement of the center of rotation, the size of the sensor 101, andthe size requirements and constraints placed upon the system by patientanatomy.

More precisely, the sensor 101 and the X-ray source 102 are aligned infront of each other according to a direction substantially parallel tothe coronal direction 208, and thus perpendicular to the mid-sagittalplane 206.

Moreover, the length of the substantially linear section is comprisedbetween 70 and 250 mm. In a general way, the length is sufficient tosweep the whole profile of the patient skull. The speed of thetranslation of the gantry 103 is typically about 4 centimeters persecond.

Of course, it will be recognized that the exposure profile may beperformed in a direction opposite from that illustrated in FIG. 2without any substantial effect on the accuracy or resolution of theimaging process.

Thus the almost linear exposure profile through the mid-sagittaldirection can provide a lateral cephalogram of the skull of the patient200.

Of course, the exposure profile depicted in FIG. 2 is only a way ofexample: a substantially linear exposure profile through the coronaldirection can provide a frontal cephalogram and a substantially linearexposure profile through a plane angled compared to the sagittaldirection can provide a tilted cephalogram.

FIGS. 3A and 3B will now be discussed. FIGS. 3A and 3B disclose a numberof active surfaces 301 to 324, which represent possible configurationsfor the active portion of the surface of the sensor of the apparatus,wherein the width of the active area varies along the height of theactive area.

The active surfaces 301 to 304 comprise two portions along theY-direction (i.e. height), each with a different width along thescanning direction X; looking at the surfaces 301 to 312, it is thusapparent that the alignment and relative dimensions of the portions mayvary considerably without straying from the scope of the invention.

For instance, in the active surfaces 301 and 302, the height of the twoportions is the same, while in the active surfaces 303 and 304 thenarrow portion is of a considerably greater height than the wideportion.

Also, in the active surfaces 302 and 304 the surface is configured to besymmetrical in its height dimension, whereas the active surfaces 301 and303 are asymmetrical.

As an example, the height of the active surfaces 301 to 312 is between120 mm and 280 mm.

The width of the narrow portion is between 2 mm and 50 mm and the widthof the wide portion is between 50 and 140 mm.

The width of the active surface will dictate the depth resolution in acorresponding portion of the image; thus, where the anatomy is scannedwith x-rays that are incident on a wide region, that part of the anatomywill be rendered with a high degree of depth resolution, and vice-versa.Thus, in the active surfaces 301-304, the lower portion of the surfacewhere the width is great is positioned to scan the parts of the skullwhere a greater depth resolution is desired (e.g. the teeth, jaw, andchin area). The rest of the skull is scanned with x-rays incident on thenarrower, upper portion of the surface, minimizing the x-ray dosage inareas where a high depth resolution is not necessary (e.g. the cranium).

In this way, the scanning is adapted to give maximum depth resolution inthe features of the patient's anatomy where such resolution is necessaryfor diagnostic et al. purposes by aligning such features with the wideportions of the surface, while minimizing x-ray dosage overall by usinga narrow portion of the surface elsewhere.

The active surfaces 305 to 308 demonstrate further possible variations.In each of these active surfaces 305-308, there are three portions: anarrow, upper portion; a wide, central portion; and a narrow, lowerportion. This takes the principle of the active surfaces 301-304 onestep further.

The upper and lower portions, where the active surfaces 305-308 arenarrow, are positioned to image the upper part of the skull and thechin/neck, respectively, while the middle portion is aligned with thepatient's jaws and teeth, irradiating them with a high dose butachieving a high depth resolution.

As with the active surfaces 301 and 303, it will be noted that theactive surfaces 305 and 307 are asymmetrical in the Y-direction, whilethe surfaces 306 and 308 are symmetrical. As appropriate, the edges ofthe portions may be aligned on a common edge, as in e.g. the activesurface 311.

It will be noted that in each of the active surfaces 305-308 the upperportion is the same width as the lower portion, and thus realizes thesame depth resolution. This need not necessarily be the case, which isillustrated by the active surfaces 309-312. In the active surfaces309-312, the lower portion is wider than the upper portion, but stillnarrower than the middle portion. As a result, the portions of theanatomy scanned by the lower portion (e.g. the chin and vertebrae) willhave a greater depth resolution than those scanned by the upper portion(e.g. the cranium and the portions of the anatomy scanned by the middleportion (e.g. the teeth) will have a depth resolution greater still. Aswith the other active surfaces, the active surfaces 309 and 311 areasymmetrical, while the active surfaces 310 and 312 are symmetrical inthe Y-direction.

FIG. 3B continues this theme, but rather than the active surfaces301-312 of FIG. 3A constructed of rectilinear shapes, the activesurfaces 313-324 shown in FIG. 3B are formed of triangular ortrapezoidal shapes. These triangular or trapezoidal shapes may bedefined by their average width, in the same way that the rectilinearportions which form the active surfaces 301-312 are defined by theirwidths.

In fact, it will be quite apparent to the person of skill in the artthat the active surfaces 313-324 correspond to the active surfaces301-312, respectively, in terms of the positioning of the portions ofthe active surface and the depth resolution achieved therein. Forexample, the active surface 322 comprises a narrow upper portion, a widemiddle portion, and a lower portion which is narrower than the middleportion but wider than the upper portion, just as does the activesurface 310. Symmetry in the Y-direction, or lack thereof, correspondsbetween the surfaces of FIGS. 3A and 3B.

Thus, by considering the active surface as a plurality of portions, andby adjusting the width of those portions according to the required depthresolution in the corresponding anatomical features to be imaged, thedose of x-rays to the patient is minimized while still maintainingsufficient depth resolution in each part of the image.

The effect of this is further illustrated in FIGS. 4 and 5. In FIG. 4,there is illustrated a skull 400, superimposed with groups of voxels402, 404, and 406. The size of the voxels 402, 404, 406 represents thedepth resolution realized in that portion of the skull 400: the narrowerthe voxel in the coronal direction, the wider the corresponding portionof the sensor surface in the scanning direction.

The voxels 402, 404, 406 each correspond to a portion of a sensorsurface, such as the active surface 312 illustrated in FIG. 3A. Sincethe upper portion of the active surface 312 is narrow, the voxel 402,oriented on the cranium, is very large. This reflects the relatively lowdepth resolution at this part of the patient's anatomy. Conversely, themiddle portion of the active surface 312 produces the voxels 404, whichare narrow and thus illustrative of the high depth resolution in thispart of the skull 400. The lower portion of the active surface 312produces the voxels 406: since the lower portion has a width betweenthat of the upper and middle portion of the surface 316, the voxels 406have a width in the coronal direction between those of the voxels 402and 404.

FIG. 5 further illustrates the difference in depth resolution, withrespect to the scan of a CBCT technique as known in the art, asillustrated by voxels 500 and 502.

The voxels 500 are representative of those produced by the imagingtechniques of the present invention. The voxel size in the Z-directionillustrates the depth resolution; the larger the voxel is in theZ-direction, the lower the depth resolution.

In the voxels 500, it is quite apparent that the voxel size varies alongthe Y-axis, in the same way as described with reference to FIG. 4. Theconcave curve of the voxels corresponds to a sensor surface such as theactive surface 306, wherein the upper and lower portions of the activesurface 306 have a narrow width and thus a low depth resolution, whilethe middle portion has a wide width and thus a high depth resolution.

In contrast, the voxels 502 illustrate a uniformly-poor depth resolutionwhen compared to the voxels 500.

FIG. 6 illustrates a collimator 600 for a scanning device. Thecollimator 600 comprises four blades 602A-602D disposed around anorifice 604 through which x-rays are projected at the patient. Each ofthe blades 602A-602D is mobile by way of a corresponding motor 606A-606Dalong an axis 608A-608D. In this way, the size and shape of the aperture604 can be adjusted according to the size and shape of the activesurface upon which the x-rays are incident during the scanning process.

In this way, x-ray leakage is minimized, in that one can be assured thatlittle to none of the x-ray energy emitted by the source escapes to thesurrounding environment.

The active surfaces (i.e. the irradiated portions of the sensor) 301-312depicted in FIG. 3A and discussed above can be properly irradiated bydisposing two successive collimators in the path of the X-ray beam. Insuch an embodiment, the aperture of the first (upstream) collimatorshapes the beam so that is has a rectangular cross-section.

However, the form of the active surface area that can be realized by asingle collimator is limited. More specifically, the collimator canreduce the area of the sensor irradiated by the X-ray beam only so longas the entire active surface is irradiated.

Thus, when the active surface is not a regular quadrilateral (e.g. theactive surface area 306 of FIG. 3A), an X-ray beam formed by a singlecollimator will irradiate the desired active surface 306, but also theportions of the sensor adjoining it, where irradiation is not desired.

The use of a second (downstream) collimator remedies this, in that itsblades are positioned to mask portions of the X-ray beam passed by thefirst collimator corresponding to the non-active portions of thesensor's surface area. In this way, the shape of the X-ray beam isfinely controlled so as to irradiate only the active surface area of thesensor, and no more.

Moreover, the second collimator may be configured such that its bladesmove in a different fashion than the first, offering a greater range ofpossible beam forms, and thus active surfaces.

Thus, in the example of the active surface 306, four blades of thesecond (downstream) collimator intercept the “corners” of therectangular X-ray beam that is passed through the first (upstream)collimator, forming the X-ray beam into the “t” shape of the activesurface 306.

The number of collimator blades employed, and the degree to which eachof them intercept the X-ray beam, may vary according to the activesurface area employed. For instance, an X-ray beam for the activesurface 311 can be formed by moving two blades of a second collimatorinto the X-ray beam: a first one from the top right which impinges theX-ray beam to a relatively large degree, and a second one from thebottom right which intercepts the X-ray beam to a relatively smalldegree. And for the active surface 301, only a single blade of thesecond collimator is necessary, intercepting the X-ray beam from thetop-right corner.

Variable-blade collimators are thus highly advantageous, in that theycan be used to form the X-ray beam according to both the size and theshape of the active surface area employed and by extension, to thepatient anatomy to be imaged, in particular the size of the skull whichdiffers greatly between children and adults.

It is also important to note that, while in the collimator 600 theblades 602A-602D move orthogonally to each other, the collimator is notnecessarily limited to such motion. Indeed, rotational motion of theblades may be envisioned, for instance where a surface such as one ofthose depicted in FIG. 3B is used. Moreover, there need not necessarilybe four blades; in fact, any number may be used as appropriate to theparticular embodiment.

For instance, where the active areas are particularly complex (such asthe active surfaces 313-324 depicted in FIG. 3B), it may be preferableto employ a combination of a single collimator downstream of a fixedaperture plate. Because it is fixed, this aperture plate may beconfigured with a more complex form than might be practical with amobile-blade collimator. By using such an aperture in conjunction with acollimator as described above, it is possible to form the X-ray beam formore complicated active areas.

FIG. 7 illustrates an exemplary configuration for the gantry 103 adaptedto displace the X-ray sensor 101 and the X-ray source 102 along theexposure profile previously described.

The gantry 103 is configured to translate laterally along the directionT, as well as to rotate about a central axis of rotation 203. In thisembodiment, the gantry 103 is mounted to the horizontal mount 104through translation and rotation means in order to be able to translateand rotate around the patient 200. For example, in order to perform theexposure profile as depicted in FIG. 2, the gantry 103 comprise atranslation mechanism configured to synchronously move the X-ray source102 and the sensor 101. In one embodiment, the translation mechanism isadapted to translate the gantry 103 with regard to the horizontal mount104 along the length of the profile exposure.

For example, in order to get a lateral cephalogram, the gantry 103 istranslated with the X-ray source 102 and the sensor 101 according to thedirection T parallel to the sagittal direction.

This rotation has a range which corresponds to the sweep angle θ asillustrated in FIG. 2.

Moreover, it can be seen in FIG. 2 that the center of rotation 203 ofthe gantry 103 moves not only in the X-direction (i.e. the sagittaldirection), but also moves slightly in the Z-direction (i.e. the coronaldirection). This is quite apparent when considering the trajectory ofthe center of rotation 203, from 203A to 203B, and from 203B to 203C. Inany case, the center of rotation 203 of the gantry 103 is positionedabove the patient's head and on the same side as the X-ray source 102(here, on the left side).

To obtain the specific exposure profile, the rotation of the X-raysensor 101 and the X-ray source 102 and the translation of the gantry103 are synchronously regulated, for example by controlling means.

The controlling means in an extra-oral dental imaging system, used tocontrol the displacement of the gantry 103 and of the X-ray sensor 101and the X-ray source 102 are known and will not be described in moredetails.

In this way, a great deal of control of the incidence of the X-ray beampassing between the X-ray source 102 and the X-ray sensor 101, relativeto the patient 200, is achieved. More specifically, the point at whichthe X-ray beam emitted by the X-ray source 102 is incident upon thepatient, as well as the angle of said beam relative to the mid-sagittalplane 206 and/or coronal plane 208 (depicted in FIG. 2) can be readilycontrolled, as the X-ray beam sweeps through the transverse plane of thepatient 200.

Ideally, the sweep angle θ will be between 15° and 30°, as such an anglepresents a good balance between patient X-ray dosage and image quality.However, in other applications, such as the imaging of other regions ofthe body, a different sweep angle θ, either wider or narrower, couldequally be envisioned.

The frame rate during the scan is comprised between 15 and 50 frames persecond, and most preferably between 15 and 30 frames per second.

High capture frame rates are generally preferable for high depthresolution; however, since the sensor 101 has a large active area andrealizes a fine depth resolution, this makes it possible to use a lowerframe rate than what might be employed in the art to reduce the amountof raw image data generated, thereby minimizing image-processingcomputing loads while still maintaining good depth resolution inre-constructed images.

Furthermore, the rotation of the gantry 103 about the axis of rotation203 permits bilateral imaging without disturbing the position of thepatient; the gantry 103 rotates through 180° to reposition the X-raysensor 101 and the X-ray source 102 to scan the patient 200 from theother side. In addition, the center of rotation 203 of the gantry 103 isdisplaced in the Z-direction so as to place the center of rotation 203on the other side of the patient's head 200, and that the patient's head200 remains in the vicinity of the sensor 101 (see FIG. 2).

This presents a considerable advantage over the cephalographictechniques presently known in the art, in that it produces two separatebilateral images, rather than one single image with superimposedanatomical figures.

Moreover, this is achieved with the same or a small increase in X-raydosage relative to the cephalographic techniques known in the art, andwith a greatly-reduced dosage relative to standard tomographic imagingtechniques which irradiate the patient from many different angles, suchas Cone Beam Computerized Tomography (CBCT).

By acquiring projections at two opposite angular extremes, separateimages for left and right sides of the patient may be obtained: two halfcephalometric images, with no superposition, are created with only twicethe X-ray's dose of a classic 2D cephalogram.

Turning now to FIG. 8, an algorithm which implements the methodaccording to an embodiment of the invention is discussed in furtherdetail.

The method comprises a step for positioning S1 of the patient 200between the X-ray source 102 and the X-ray sensor 101. This is ideallyaccomplished by the use of a headrest or brace as the patient holder 106displayed in FIGS. 1A and 1B, so as to create a proper reference pointupon which the data extracted during the scan can be constructed into animage.

A setting step S2 is performed in order to set up the active area 300 ofthe X-ray sensor 101. As known in prior art, a collimator is used infront of the X-ray source 102 in order to adapt the X-ray beam to theactive area 300 of the X-ray sensor 101.

As previously described and depicted in FIGS. 3A and 3B, this activearea is determined in order to have a width varying along the height ofthe active area.

Once the patient is positioned, there is an activating step S3 foractivating the X-ray source, followed by a displacing step S4 forsweeping the X-ray beam across the patient. The displacing step S4involves both synchronously translating and rotating the X-ray source102 and X-ray sensor 101, thereby sweeping the beam across the patientas depicted in FIG. 2.

The multiple frames produced by the X-ray sensor 101 during the exposureof the skull are registered during a registering step S5.

Optionally, a test S6 is conducted in order to determine if an oppositescan is needed.

In the affirmative, a step S7 for repositioning the X-ray source 102 andX-ray sensor 101 is performed, in which the X-ray source 102 and sensor101 are rotated through 180° about the patient 200, and the center ofrotation 203 displaced in the Z-direction. In this way, the steps ofactivating S3, displacing S4 and registering S5 for a second scan of thepatient 200 are achieved, which are performed in substantially the sameway as described above with relation to steps S3 to S5.

Thus, in this embodiment the exposure profile comprises two separatesubstantially linear sections at two opposite angular extremes about thepatient 200.

Once the scan(s) is (are) complete, the method reconstructs the datagathered by the X-ray sensor 101 into a useable image, forreconstructing three-dimensional volumetric data of the patient anatomy.

For this, the method comprises a step S8 of computing the multipleframes produced during at least part of the exposure. This computingstep S8 involves applying a shift-and-add processing to reconstruct theimage as a series of “slices,” each containing a small portion ofvolumetric data describing the patient's anatomy.

Apart from tomosynthesis, other 3D reconstruction techniques can also beemployed. The use of an iterative reconstruction algorithm such as SART(Statistical Algebraic Reconstruction Technique) or SIRT (StatisticalIterative Reconstruction Technique), or a technique like a Filtered BackProjection, to obtain the volumetric data opens up the possibility ofobtaining an artifact-free volume with a very low x-ray dose. TheDigital Cephalometric Tomosynthesis can be done with a dose between oneand two times that of a classic 2D Cephalogram.

Generally, a single plane or the complete volume can be reconstructedusing the Shift & Add, Filtered Back Projection, or Iterativetechniques.

The Shift & Add algorithm is the fastest method, reconstructing adesired plane or set of planes according to the acquisition trajectoryand desired anatomy. The Shift & Add algorithm does require theapplication of a de-blurring or enhancement filter to the reconstructedimage, but is overall economic with computing resources.

The Filtered Back Projection technique is an approach similar to Shift &Add, but with the added possibility of obtaining a complete 3D volume.From this volume views along certain desired planes can then beextracted.

The iterative approaches (e.g. SIRT & SART) use some a prioriinformation about the object, in particular, anatomical a prioriinformation such as selected cranial measurements and/or the limits ofthe skull in space. The use of a priori information such as these canhelp compensate for limited angle problems, as well as help reduce thenecessary x-ray dosage. However, iterative reconstruction methods aregenerally slower than other methods.

The reconstructed slices contain in focus imaging data belongingrespectively to several depths of the imaged skull.

On the one hand, each slice can be used for the extraction of somecephalometric features. As depicted in FIG. 9, the in focus featuresextracted from a slice on the mid-sagittal plane of a human skull may beobtained.

Moreover, the extracted features through different depths can be puttogether to provide a cephalometric image.

As shown in FIG. 10, the superimposition of at least some of the infocus features extracted from all slices belonging to a left part of ahuman skull provides an half cephalometric image for the left side ofthe patient.

Such a reconstruction is particularly advantageous, in that unlike themethods known in the art, the reconstruction is not limited to a narrowfocal trough in which the image is clear and undistorted, but rathereach aspect of the patient's anatomy is represented by the slice inwhich that aspect is displayed with the highest fidelity and resolution.Such a method will ultimately produce an accurate, three-dimensionalmodel of the patient's anatomy while simultaneously minimizing X-raydosage.

A consequence of this is that a great deal of other, more conventionalimages can be simulated through simple data processing methods, based onthe model produced in the step S8 for reconstructing the 3D volumetricdata.

For instance, simulated left and right 2D lateral images, eitherseparate or superimposed, can be constructed by passing the 3D modelthrough a step S9 for retro-projecting, i.e. simulating a standardcephalogram as produced by a “ceph arm”-equipped imaging system throughknown rendering techniques such as cone beam or parallel geometryprojection.

Such a retro-projection might be calibrated to produce a simulatedretro-projection distance of between 1.5 m and 4 m, so as to bestsimulate the images produced by current imaging systems; however it willbe understood that other projection distances might also be advantageousin other applications, and that the method of the present invention caneasily be so adapted. An infinite projection distance is particularlyadvantageous, in that it corresponds to a parallel beam projection.

The synthetized 2D cephalogram looks like a standard cephalogram asobtained with a X-ray sensor located on a cephalometric arm of 1.50 m ormore related to the position of the X-ray source.

Producing a panoramic image of the mandible and dentition may also beenvisioned, performed in a similar way using other, appropriaterendering methods.

These extracted, simulated images can in themselves have importantdiagnostic and therapeutic uses.

For instance, by extracting the relevant cephalometric features from the3D model and/or 2D images, and identifying them on said 2D lateralimages in an extraction step S10, a cephalometric tracing can beproduced at an automatic cephalometric tracing step S11. Such acephalometric tracing yields valuable diagnostic and therapeutic data,and is much easier and faster to produce than the plaster-moldingmethods presently employed.

FIG. 11 illustrates a possible variant of the invention. In FIG. 11,there is provided a scanning apparatus 800 comprising a sensor 802,which is employed for imaging a patient 804.

The sensor 802 is configured according to the above description, in thatit has different regions of varying width along its height. For thepurposes of illustration, it should be considered that in thisembodiment the sensor 802 has an active surface substantially similar inform to the surface 302 of FIG. 3A. However, sensors with activesurfaces in other forms may also be envisioned.

With such a sensor it is possible to scan the entirety of the patient'shead 804 in one pass, such as by a centrally-located X-ray source 806,as in the other embodiments of the invention discussed above.

In the present embodiment, however, a different approach is employed.Here, an X-ray source 808 is disposed in a first, upper position, fromwhich it irradiates an upper portion of the patient's skull 804 with abeam 810 of X-rays. This beam 810 illuminates only, or at leastprimarily, the upper portion of the sensor 802, which has a reducedwidth in the scanning direction as a consequence of the lower degree ofdepth resolution required to sufficiently image the patient's cranium.

Once this is accomplished, the X-ray source 808 is moved into the lowerposition, where the scan is repeated. Here, however, the X-ray beamirradiates the lower portion of the skull.

As a result, the X-rays passing through the patient 804 illuminate only,or at least primarily, the lower portion of the sensor 802, which iswider in the scanning direction than the upper portion and thus has agreater depth resolution. In this way, the lower portion of the skull,containing the mandibles, dentition, and other such complex structures,is imaged at high resolution.

The scanning steps may optionally be repeated from the other side of thepatient's head 804. There may also be scans performed with the X-raysource 808 disposed in one or more intermediate positions, between theupper and lower positions depicted in FIG. 11, each intermediateposition possibly corresponding to a region or regions of the activearea of the sensor 802.

Such a method is particularly advantageous in that it allows for a moregranular control of the X-ray dose received by the patient 804: inparticular, the X-ray beam 810 may be more or less intense, depending onwhich portion of the patient's anatomy is being scanned and whichportion of the active surface of the sensor 802 is being illuminated. Inturn, this results in a further refinement of image quality, with anX-ray dosage that, while being slightly greater than other embodimentsof the present invention, is nonetheless considerably reduced comparedto the techniques known in the art.

In any case, the resulting images are combined, such as by stitching orother such image processing techniques, so as to create a finishedthree-dimensional scan of the patient 804.

1. A method for creating a cephalometric image of at least part of ahuman skull in an extra-oral dental imaging system, said systemcomprising: an X-ray source for irradiating an object to be imaged; animaging device suitable for producing multiple frames during at leastpart of an exposure of said object; a manipulator for displacing theimaging device along an exposure profile between multiple frames duringsaid at least part of the exposure of said object, the manipulatorpermitting the movement of the imaging device along a scanning direction(X), characterized in that said method comprises the following steps:setting said imaging device with an active area having in an imagingplane a width, extending along said scanning direction (X), said widthvarying along a height direction (Y) perpendicular to said scanningdirection (X); synchronously displacing the X-ray source and the imagingdevice along said exposure profile; and registering said multiple framesproduced by the imaging device during the exposure of said object to beimaged.
 2. A method according to claim 1, wherein said active area issymmetric in said imaging plane, with a central axis extending alongsaid height direction (Y) of said active area.
 3. A method according toclaim 1, wherein said active area has at least two portions havingwidths different from each other, said two portions being superposed insaid imaging plane along said height direction (Y).
 4. A methodaccording to claim 3, wherein the height of said active area is between120 mm and 280 mm, the width of a first portion is between 2 mm and 50mm, and the width of a second portion is between 50 and 140 mm.
 5. Amethod according to claim 1, wherein said active area has at least threeportions with respectively three different widths.
 6. A method accordingto claim 1, wherein said active area has a central portion, a lowerportion, and an upper portion each extending along said height direction(Y) in said imaging plane, the width of the central portion is largerthan the width of the lower portion and the width of the lower portionis larger than the width of the upper portion.
 7. A method according toclaim 1, further comprising a step of computing said multiple framesproduced during at least one part of the exposure: by a shift-and-addprocessing, thereby reconstructing at least one slice; or by avolumetric approach, thereby reconstructing a three-dimensional volumeand subsequently extracting at least one slice from this volume; whereinsaid at least one slice includes in-focus imaging data belongingrespectively to at least one depth of said object to be imaged.
 8. Amethod according to claim 7, wherein the volumetric approach is selectedfrom a Statistical Algebraic Reconstruction Technique (SART), aStatistical Iterative Reconstruction Technique (SIRT), or a FilteredBack Projection technique.
 9. A method according to claim 7, furthercomprising a step of using each reconstructed slice for the extractionof cephalometric features.
 10. A method according to claim 9, furthercomprising a step of automatic cephalometric tracing, wherein saidextracted cephalometric features of each slice are put together.
 11. Amethod according to claim 9, wherein several slices are reconstructedand combined to give a separate linear projection for the left and rightsides of said object to be imaged.
 12. A method according to claim 7,wherein several slices are reconstructed and retro projected to adistance superior to 1.50 meters, and preferably superior to 4 meters,on a cone beam or parallel geometry so as to create a synthesized 2Dcephalogram of the skull.
 13. A method according to claim 1, comprisingthe following steps: synchronously displacing the X-ray source and theimaging device along a first part of said exposure profile, said X-raysource being in an upper position along said height direction (Y);registering said multiple frames produced by the imaging device duringsaid first part of the exposure profile; synchronously displacing theX-ray source and the imaging device along a second part of said exposureprofile, said X-ray source being in a lower position along said heightdirection (Y); registering said multiple frames produced by the imagingdevice during said second part of the exposure profile; and combiningsaid multiple frames registered during said first and second parts ofthe exposure profile.
 14. An extra-oral dental imaging system forcreating a cephalometric image of at least part of a human skull, suchsystem comprising: an X-ray source for irradiating an object to beimaged; an imaging device suitable for producing multiple frames duringat least part of an exposure of said object; manipulator for displacingthe imaging device along an exposure profile between multiple framesduring said at least part of the exposure of said object, themanipulator permitting the movement of the imaging device along ascanning direction (X), characterized in that said imaging device has anactive area having in an imaging plane a width extending along saidscanning direction (X), said width varying along a height direction (Y)perpendicular to said scanning direction (X); and in that saidmanipulator synchronously displaces the X-ray source and the imagingdevice along said exposure profile; and in that it comprises memory forregistering said multiple frames produced by the imaging device duringthe exposure of said object to be imaged.